The present invention generally relates to an electro-hydraulic braking system in which the braking force acting upon a service brake is generated by an electrically controlled pump.
A simplified version of this system is shown in FIG. 1. This system 10 comprises a pump 12 powered by a motor 14 in response to control signals by an electric control unit (ECU) 16. The pump 12 directly pressurizes a brake cylinder 18. The speed of the motor 14/pump 12 is controlled to modulate brake system pressure in accordance with a commanded brake pressure signal. In order for the brake system pressure to decay or decrease quickly, fluid within the brake system must flow to a drain or reservoir 22. The speed of the pressure decay can of course be increased dramatically if the pump direction is reversed, however, this adds to the complexity of the system. As can be appreciated, when the pump speed is slowed or when the pump is brought to a halt, brake system pressure will decay through the internal leak passages 24 of the pump, however, the time required for such decay is relatively long. One solution is to add an orifice 26 (an additional leak path) across the pump 12 to speed the brake pressure decay. This approach requires that the pump capacity or displacement be increased since the pump now must be sized to supply the leakage flow as well as to supply fluid to adequately pressurize the brake cylinder 18. If a fixed orifice 26 is used, it must be large enough to provide a quick decay of the brakes when the motor is turned off. A large orifice will however require very large flows to generate high pressures and would require an unreasonably high power output motor/pump system.
It is an object of the present invention to provide an improved electro-hydraulic braking system.
Accordingly, the invention comprises: a brake system comprising: a master cylinder and a pump for selectively pressurizing an associated wheel cylinder or cylinders. An isolation valve may be provided to provide the selective communication. The system including a motor for powering the pump and first means for modulating motor speed in response to a signal indicative of desired braking effort and actual braking effort. A control valve is provided in circuit with the pump and wheel cylinder. The valve is responsive to master cylinder pressure to control the flow area in the path to a reservoir. By reducing the flow area at high master cylinder pressure a smaller pump can be used. Additionally, when the master cylinder pressure is decreased, the control valve increases the flow area to the reservoir allowing for a rapid pressure decay of the brakes.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.